An example of a known hybrid vehicle driving apparatus where an engine and a motor generator drive vehicle wheels is disclosed in JP2008-55993A, hereinafter referred to as Reference 1. The hybrid vehicle driving apparatus disclosed in Reference 1 includes an engine, a front clutch, a motor generator, and an automatic transmission being connected in series to provide a vehicle with the driving apparatus simultaneously using the engine and the motor generator.
The driving apparatus disclosed in Reference 1, for example, drives vehicle wheels by the motor generator when the vehicle starts moving from a stopped state. In a state where the vehicle in a running state accelerates, driving power from the engine is added to the vehicle wheels by engaging the front clutch arranged between the engine and the motor generator.
In a state where the vehicle in the running state is decelerated by providing a brake operation, the driving apparatus disclosed in Reference 1 operates a regenerative brake at the motor generator with the front clutch disengaged in order to prevent a regenerative energy being reduced by the engine rotation. The driving apparatus disclosed in Reference 1 operates the front clutch to selectively engage the engine and the motor generator depending on a situation. In a state where the front clutch engages the engine and the motor generator, an engine speed is synchronized with a motor generator speed and then transmission torque of the front clutch is gradually increased in order to reduce a shock that accompanies engagement of the front clutch.
Meanwhile, the automatic transmission shifts gears in accordance with the driving state of the vehicle acquired from an accelerator position and a vehicle speed. An example of such automatic transmission is disclosed in Japanese patent number 3902044B, hereinafter referred to as Reference 2. The automatic transmission disclosed in Reference 2 engages or disengages a multiple number of friction clutches connected to a multiple number of planetary gears to shift gears. More specifically, the automatic transmission shifts gears by reducing hydraulic pressure at the friction clutch to be disengaged of current gear shift stage to reduce transmission torque of the friction clutch to be disengaged, and by gradually increasing transmission torque of the friction clutch to be engaged of next gear shift stage by increasing hydraulic pressure at the friction clutch to be engaged.
In such automatic transmissions, a delay in reducing transmission torque of the friction clutch to be disengaged relative to increasing transmission torque of the friction clutch to be engaged results in a state known as an interlock state, which results in rapidly losing output torque. Being too fast in reducing transmission torque of the friction clutch to be disengaged relative to increasing transmission torque of the friction clutch to be engaged results in rapidly losing output torque as a result of rapid decrease of torque transmitted from an input shaft to an output shaft. In each case, a shift shock occurs, the shift shock that is considered as a decrease in shift quality.
An example of a known method to maintain shift quality of gear shift, which is described in Reference 2, is to reduce transmission torque of the friction clutch to be disengaged to generate a predetermined amount of slip at the input shaft and holding the friction clutch to be disengaged at that state until the transmission torque at the friction clutch to be engaged to increase. In order to prevent the shift shock, transmission torque of the friction clutch to be disengaged is controlled to provide the predetermined amount of slip at the input shaft, which corresponds to an increasing amount of rotation numbers. This method of controlling the transmission torque of the friction clutch to be disengaged is known as a slip amount feed back control.
In each of the hybrid vehicles in Reference 1 and Reference 2, the front clutch and the automatic transmission operate independently. As a result, in a situation where the front clutch is operated while gear shift at the automatic transmission is in progress, an input torque provided as an input at the input shaft of the automatic transmission fluctuates. The fluctuation of input at the input shaft of the automatic transmission affects the slip amount feed back control as an increase of the shift shock during gear shift at the automatic transmission.
A need thus exists for a hybrid vehicle driving apparatus, which is not susceptible to the drawback mentioned above.